Step 1: Tools

Step 2: Disassembly

First take your side cutters and carefully remove brass valve on the top of cartridge.

Step 3:

Using a small sheet metal screw carefully thread it into the one way check valve in cartridge.

Step 4:

Once screw is secure use side cutters to carefully "wiggle" and pull one way check valve out.

Step 5:

Firmly snap the first valve back in. It should "snap" when in.

Step 6:

Ok now set the "prepped" cartridge aside for now.
Grab the butane refill and remove the tapered end on the top of the can.
Now using a sharp knife of your choice slit the plastic nozzle end. Make the slit somewhat tapered so when it's pressed into the cartridge valve it can squeeze together. (This might take some fine tuning to get just right)

Step 7:

Last thing, hold empty cartridge upright and refill with the butane canister. The fill amount does sometimes vary from 1/3 to almost full.
Now mark the butane canister tip so you can replace it when you need to buy a new refill, you won't have to cut a new tip. Also mark your "hacked" thermacell cartridges so you know which ones are refillable and which ones are not